diff --git a/iwyu.cc b/iwyu.cc
index 3d42646..ea2ec49 100644
--- a/iwyu.cc
+++ b/iwyu.cc
@@ -1484,6 +1484,8 @@ class IwyuBaseAstVisitor : public BaseAstVisitor<Derived> {
               GetFileEntry(call_expr), GetFileEntry(*fn_redecl))) {
         continue;
       }
+      if (fn_redecl->isThisDeclarationADefinition() && !IsInHeader(*fn_redecl))
+        continue;
       for (set<const Type*>::iterator it = retval.begin();
            it != retval.end(); ) {
         if (!CodeAuthorWantsJustAForwardDeclare(*it, GetLocation(*fn_redecl))) {
@@ -1768,6 +1770,11 @@ class IwyuBaseAstVisitor : public BaseAstVisitor<Derived> {
         while ((redecl = redecl->getPreviousDecl()))
           ReportDeclUse(CurrentLoc(), redecl);
       }
+      if (!IsInHeader(decl)) {
+        // No point in author-intent analysis of function definitions
+        // in source files.
+        return true;
+      }
     } else {
       // Make all our types forward-declarable...
       current_ast_node()->set_in_forward_declare_context(true);
diff --git a/iwyu_location_util.cc b/iwyu_location_util.cc
index 31374ad..4036156 100644
--- a/iwyu_location_util.cc
+++ b/iwyu_location_util.cc
@@ -10,6 +10,7 @@
 #include "iwyu_location_util.h"
 
 #include "iwyu_ast_util.h"
+#include "iwyu_port.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclCXX.h"
@@ -28,6 +29,7 @@ using clang::CXXMethodDecl;
 using clang::CXXOperatorCallExpr;
 using clang::ClassTemplateSpecializationDecl;
 using clang::ConditionalOperator;
+using clang::FileEntry;
 using clang::FunctionDecl;
 using clang::MemberExpr;
 using clang::SourceLocation;
@@ -170,4 +172,10 @@ bool IsInScratchSpace(SourceLocation loc) {
   return StartsWith(PrintableLoc(GetSpellingLoc(loc)), "<scratch space>");
 }
 
+bool IsInHeader(const clang::Decl* decl) {
+  const FileEntry* containing_file = GetFileEntry(decl);
+  CHECK_(containing_file);
+  return !GlobalSourceManager()->isMainFile(*containing_file);
+}
+
 }  // namespace include_what_you_use
diff --git a/iwyu_location_util.h b/iwyu_location_util.h
index 6f8cf81..a875ed8 100644
--- a/iwyu_location_util.h
+++ b/iwyu_location_util.h
@@ -228,6 +228,9 @@ inline bool IsBeforeInSameFile(const T& a, const U& b) {
   return IsBeforeInTranslationUnit(a, b);
 }
 
+// Returns true if the given declaration is located in a header file.
+bool IsInHeader(const clang::Decl*);
+
 }  // namespace include_what_you_use
 
 #endif  // INCLUDE_WHAT_YOU_USE_IWYU_LOCATION_UTIL_H_
diff --git a/tests/cxx/implicit_ctor-d1.h b/tests/cxx/implicit_ctor-d1.h
index aa9c4cf..a5aa209 100644
--- a/tests/cxx/implicit_ctor-d1.h
+++ b/tests/cxx/implicit_ctor-d1.h
@@ -22,6 +22,14 @@ int ImplicitCtorFn(IndirectWithImplicitCtor);
 // IWYU: IndirectWithImplicitCtor is...*implicit_ctor-i2.h.*for autocast
 int ImplicitCtorRefFn(const IndirectWithImplicitCtor&);
 
+// Reporting types for "autocast" for header-defined functions still makes sense
+// as opposed to function definitions in source files.
+// IWYU: IndirectWithImplicitCtor needs a declaration
+// IWYU: IndirectWithImplicitCtor is...*implicit_ctor-i2.h.*for autocast
+inline int InlineImplicitCtorRefFn(const IndirectWithImplicitCtor&) {
+  return 1;
+}
+
 // Test parameter type uses that do not require special handling for "autocast".
 int NoAutocastFn(
     // A subtle c++ point: forward-declaring is ok for nonconst, because
diff --git a/tests/cxx/implicit_ctor.cc b/tests/cxx/implicit_ctor.cc
index bb5341b..2c088a0 100644
--- a/tests/cxx/implicit_ctor.cc
+++ b/tests/cxx/implicit_ctor.cc
@@ -22,6 +22,18 @@
 
 #include "tests/cxx/implicit_ctor-d1.h"
 
+// No reporting types for "autocast" for .cpp-file-local functions...
+// IWYU: IndirectWithImplicitCtor needs a declaration
+static int LocalFn(const IndirectWithImplicitCtor&) {
+  return 1;
+}
+
+// ... or for .cpp-file-definitions of functions declared in a header.
+// IWYU: IndirectWithImplicitCtor needs a declaration
+int ImplicitCtorRefFn(const IndirectWithImplicitCtor&) {
+  return 2;
+}
+
 // We don't need IndirectWithImplicitCtor even though we must convert to it.
 int a = ImplicitCtorFn(1);
 int b = ImplicitCtorRefFn(2);
@@ -32,6 +44,13 @@ int c = ImplicitCtorFn(IndirectWithImplicitCtor(3));
 // IWYU: IndirectWithImplicitCtor is...*implicit_ctor-i2.h
 int d = ImplicitCtorRefFn(IndirectWithImplicitCtor(4));
 
+// LocalFn doesn't provide IndirectWithImplicitCtor type info.
+// IWYU: IndirectWithImplicitCtor is...*implicit_ctor-i2.h
+int e = LocalFn(5);
+// InlineImplicitCtorRefFn should provide parameter type info,
+// hence no reporting.
+int f = InlineImplicitCtorRefFn(6);
+
 // Make sure we are responsible for the conversion when it's not for a
 // function call.
 // IWYU: IndirectWithImplicitCtor is...*implicit_ctor-i2.h
@@ -48,7 +67,7 @@ tests/cxx/implicit_ctor.cc should add these lines:
 tests/cxx/implicit_ctor.cc should remove these lines:
 
 The full include-list for tests/cxx/implicit_ctor.cc:
-#include "tests/cxx/implicit_ctor-d1.h"  // for ImplicitCtorFn, ImplicitCtorRefFn
+#include "tests/cxx/implicit_ctor-d1.h"  // for ImplicitCtorFn, ImplicitCtorRefFn, InlineImplicitCtorRefFn
 #include "tests/cxx/implicit_ctor-i2.h"  // for IndirectWithImplicitCtor
 
 ***** IWYU_SUMMARY */